1. Field of the Invention
The present invention relates to a method for forming a silicon semiconductor thin film which has a crystallinity and is formed on a substrate having an insulation surface of a glass substrate or the like.
2. Description of the Related Art
In recent years, attention is paid on a technology for constructing a thin film transistor by using a silicon thin film which is formed on a glass substrate. This thin film transistor is primarily used in an active matrix type liquid crystal electro-optical device and other thin film integrated circuits. The liquid crystal electro-optical device changes optical characteristics of a liquid crystal thereby displaying an image by charging a liquid crystal into a pair of glass substrates and applying an electric field.
In particular, the active matrix type liquid crystal display device using a thin film transistor is characterized in that the thin film transistor is arranged in each pixel, an electric charge held in a pixel electrode is controlled by using the thin film transistor as a switch. Since the active matrix type liquid crystal display device is capable of displaying a fine image at a high speed, the device can be used in displays for various electronic apparatuses (for example, a portable word processor, and a portable computer or the like).
As a thin film transistor used in the active matrix type liquid crystal display device, an amorphous silicon thin film is commonly used. However, the thin film transistor using the amorphous silicon thin film has the following problems.
(1) The liquid crystal thin film transistor has low characteristics and cannot display a higher quality image.
(2) The liquid crystal thin film transistor cannot constitute a peripheral circuit for driving a thin film transistor arranged on a pixel.
The aforementioned second problem can be considered by dividing the problem into the following two aspects. One aspect of the problem is that since a P-channel type thin film transistor cannot be used for practical purposes with the thin film transistor using an amorphous silicon thin film, a CMOS circuit cannot be constituted. Another aspect of the problem is that since the thin film transistor using an amorphous silicon thin film cannot be operated at a high speed, and a large current cannot flow in the thin film transistor, a peripheral driving circuit cannot be assembled.
Means for solving the aforementioned problems include a technology for forming a thin film transistor by using a crystalline silicon thin film. The methods for obtaining a crystalline thin film include a method for heat treating an amorphous silicon film and a method for irradiating the amorphous silicon thin film with laser light.
However, a method for crystallizing am amorphous silicon film by heat treatment in the prior art has the following problem.
In the case where a thin film transistor is constituted which is used in a liquid crystal electro-optical apparatus, it is demanded that the thin film transistor is formed on a translucent substrate. Examples of the translucent substrate include a quartz substrate and a glass substrate. However, the quartz substrate is expensive and cannot be used in the liquid crystal electro-optical device which has a large technological problem of a cost reduction. Consequently, the glass substrate is commonly used. However, it has a problem of a low heat resistance.
Generally, as the glass substrate used in the liquid crystal electro-optical device, Corning 7059 glass substrate is used. The strain point of this glass substrate is 593xc2x0 C. When the substrate is heat treated at this temperature or more, the shrinkage or the deformation of the substrate becomes conspicuous. In recent years, the liquid crystal electro-optical device tends to have a larger area and the shrinkage and the deformation of the substrate must be suppressed as much as possible.
However, it has been proved in the experiment that a temperature of 600xc2x0 C. or more is required to crystallize the amorphous silicon film by heating. It is also made clear that tens of hours are required for heating. A large area glass substrate cannot be subjected to such high temperature and long hour heating at all.
Further, a technology of crystallizing the amorphous silicon film by laser light irradiation is also known. However, it is difficult as a practical problem to irradiate uniformly a large area with laser light and to irradiate the area while keeping a definite level of irradiation power.
An object of the present invention is to solve the aforementioned problems and to provide a technology of transforming the amorphous silicon film into a crystalline silicon film by heat treatment at an extremely low temperature.
In particular, an object of the invention is to provide a crystalline silicon thin film which is capable of constituting a thin film transistor with a high performance characteristics.
In accordance with a major aspect of the present invention, there is provided a method for fabricating a semiconductor thin film comprising the steps of:
introducing into an amorphous silicon film a metal element which promotes the crystallization of silicon;
obtaining a crystalline silicon film by crystallizing the aforementioned amorphous silicon film by heat treatment;
forming a metal element diffusion film on said crystalline silicon film;
diffusing the aforementioned metal element into the aforementioned metal element diffusion film; and
removing the metal element diffusion film into which the metal element has been diffused.
In the aforementioned structure, examples of the amorphous silicon film include a film which is formed by the plasma CVD or by the low pressure thermal CVD on a glass substrate or on a glass substrate on which an insulating film is formed.
Further, examples of the metal element which promotes the crystallization of the aforementioned silicon include one or more kinds of elements selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au. Among these metal elements, the most effective metal element is nickel (Ni).
Methods for introducing the metal element which promotes the crystallization of the aforementioned silicon include a method for providing these metal layers or a layer including the metal on the surface of the amorphous silicon film. Specifically, the methods include a method for forming a metal element layer or a layer including a metal element by the CVD, the sputtering process, vapor deposition or the like, and a method for coating a solution containing a metal element on the amorphous silicon film. In particular, since the latter method using the latter solution enables easily controlling a density of the metal element, the latter method is more favorable than the former method. In addition, since the metal element can be held uniformly in contact with the surface of the amorphous silicon film in the latter method, the latter method is very favorable in this respect, too. For reference, when the aforementioned CVD and the sputtering process, vapor deposition or the like is used, it is difficult to form an extremely thin uniform film. Consequently, there is a problem in that the metal element is non-uniformly present on the amorphous silicon film, and the metal element is liable to be deviated at the time of the crystal growth.
To crystallize by heating the silicon film to which the metal element promoting the crystallization of silicon is introduced, the silicon film may be heated at a temperature of 450xc2x0 C. or more. The upper limit of this heating temperature is limited by the heat-resistant temperature of the glass substrate used as the substrate. In the case of the glass substrate, the heat resistant temperature can be regarded as the strain point of glass. When materials such as quartz substrate or the like which can endure a temperature of 1000xc2x0 C. or more, the heating temperature in heating can be heightened in accordance with the heat insulating temperature.
As one example of heat treatment, it is appropriate to set the temperature to about 550xc2x0 C. from the viewpoint of the heat resistance and the productivity of the glass substrate.
An amorphous silicon film used as the metal element diffusion film and formed on crystalline silicon film crystallized as a result of heat treatment may be formed by the general CVD process. For example, the same method as used for forming the crystalline silicon film by introducing the metal element into the amorphous silicon starting film and crystallizing the amorphous silicon starting film by heating is used.
However, more preferably, the film may be of the quality such that a defect density is high and the metal element can be easily trapped. This is because the metal element in the crystalline silicon film can be easily dispersed in the metal element diffusion film comprising silicon.
The defect density can be set to a high level by using such means as forming the film only of silane without using. hydrogen in accordance with the plasma CVD, using the sputtering method, or lowering the temperature at which the film is formed in accordance with the plasma CVD.
It is more advantageous to increase the thickness of this amorphous silicon film with respect to the thickness of the crystalline silicon film. This is because when the amorphous silicon film is thicker than the crystalline silicon film, the volume ratio with respect to the crystalline silicon film can be increased, and more metal elements can be dispersed in the amorphous silicon film.
A polycrystalline silicon film and an amorphous SixGi1-x film (0 less than x less than 1) can be used as the metal element diffusion film. The polycrystalline silicon film can be formed by low pressure CVD. The amorphous SixGe1-x film can be formed by plasma CVD using silane (SiH4) and germane (GeH4) as a raw material gas.
The step of diffusing (absorbing) the metal element in the crystalline silicon film is carried out by heat treatment. Since the metal element in the crystalline silicon film is diffused into the metal element diffusion film by the heat treatment, the metal element concentration is lowered in the crystalline silicon film.
Next, the metal element diffusion film into which the metal element has been diffused is removed. This removal can be a selective etching of the metal element diffusion film by forming an oxide film as an etching stopper on the silicon film to be crystallized.
Concrete constitution of this method is described using FIG. 1. First, a crystalline silicon film 105 is formed on a glass substrate 101 using nickel as a metal element which promotes crystallization of silicon. Reference numeral 102 designates a base silicon oxide film. Heat treatment is used for the crystallization. (FIG. 1(B))
Next as shown in FIG. 1(C), an oxide film 106 is formed, and an amorphous silicon film 107 is formed as the metal element diffusion film and heat treated.
This heat treatment can be classified into two methods; one method is the one which is carried out at a temperature (generally, 450xc2x0 C. or less) at which the amorphous silicon film is not crystallized while the other method is one which is carried out at a temperature (generally, 450xc2x0 C. or more, and preferably 500xc2x0 C. or more) at which the amorphous silicon film is crystallized.
When the heat treatment is carried out at a temperature at which the amorphous silicon film 107 provided on the crystalline silicon film 105 is not crystallized, the temperature of the heat treatment is 400xc2x0 C. to 450xc2x0 C. and the heating duration is 5 minutes to 10 hours. The metal element in the amorphous silicon film is gradually absorbed into the amorphous silicon film 107 by this heat treatment. Consequently, when the heat treatment is carried out over a long period, the density of the metal element in the crystalline silicon film 105 can be gradually decreased.
By removing the amorphous silicon film 107 using the oxide film 106 as an etching stopper, a crystalline silicon film 108 containing a metal element at a low concentration therein as compared with the metal element concentration in the amorphous silicon film 107 can be obtained. (FIG. 1(D)) This is because the silicon atom is present in the amorphous silicon film 107 in the state that the silicon atom is liable to be connected with the metal element (in the amorphous state, a large quantity of unpaired bonds are present). Further, this action can be conspicuously obtained when the defect density is artificially increased in the amorphous silicon film 107.
In the meantime, when the amorphous silicon film is heated at a temperature at which the crystallization of the amorphous silicon film 107 provided on the crystalline silicon film 105 proceeds, the dispersion of the metal element is ostensibly suspended in the state in which the amorphous silicon 107 is crystallized. Then when the average value of the density of the metal element in the crystalline silicon film 105 and the density of the metal element in the silicon film 107 for absorbing the metal element (crystallized in heat treatment) become approximately equal to each other, the dispersion of the metal element is ostensibly suspended.
However, it has been made clear that the metal element is locally concentrated in the crystalline silicon film 105. This method becomes effective for suppressing this phenomenon. Therefore, this is a method which is intended to produce a state in which no concentration of the metal element exists in the silicon film to be used in the fabrication of the device by using the phenomenon that the metal element is concentrated on the tip part of the crystal growth to dispel the tip of the crystal growth to the silicon film to be removed later.
Then, heat treatment is carried out at the temperature at which the amorphous silicon film 107 is crystallized to crystallize the amorphous silicon film 107. At this occasion, the crystal growth proceeds from a surface at which the silicon film 107 contacts the-oxide film 106 to the exposed surface thereof. Then, at the same time with this crystal growth, a portion where the metal element is concentrated moves in the silicon film 107. As a consequence, a portion where a nickel element is concentrated is dispelled from the silicon film 105 so that the nickel element exists in the silicon film 107 (particularly on the surface thereof). Then, a crystalline silicon film 108 free from an area where the nickel element is deviated can be obtained by removing the silicon film 107 using the oxide film 106 as an etching stopper (FIG. 1(D)).
An amorphous silicon film is formed on the surface of the crystalline silicon film crystallized with the action of metal which promotes the crystallization, followed by heat treatment to disperse the metal element into the amorphous silicon film. In this manner, the metal element in the crystalline silicon film can be virtually sucked out thereby making it possible to obtain a crystalline silicon film having a low density of metal element and having a crystallinity.
In addition, since all these steps can be carried out at a temperature such as 550xc2x0 C. or less which the glass substrate can endure, the steps are extremely useful in the formation of a crystalline thin film silicon semiconductor used for constructing a thin film transistor used in a liquid crystal electro-optical device using, for example, a glass substrate.
To facilitate the removal of the silicon film for the absorption of the metal element, it is effective to form an oxide film on the crystalline silicon film. Since the oxide film has a selectivity with respect to an etchant (for example, hydradine and ClF3), the oxide film can serve as an etching stopper.
Further, on a surface of the crystalline silicon film (first silicon film) crystallized with the action of the metal promoting the crystallization, an amorphous silicon film (second silicon film) is formed. Thereafter, heat treatment is carried out to crystallize the second layer amorphous silicon film with the result that the concentrated portion of the metal element which is present in the first silicon film can be dispelled into the second silicon film thereby suppressing the deviation of the metal element in the first silicon film.